This invention relates to devices for monitoring vital signs such as blood pressure, pulse rate, and oxygen saturation. The invention has particular application in devices for determining blood pressure by measuring a pulse wave velocity or pulse transit time.
Equipment for monitoring the vital signs of subjects is widely used in clinical settings. Such devices may monitor various physiological signs including blood pressure, oxygen saturation, pulse rate and the like. Such devices typically include one or more sensors placed at suitable locations on the subject""s body. Various different types of sensors may be used. The sensors may be of invasive types or of non-invasive types. Signals from the sensors are carried to the vital signs monitoring equipment where they are amplified, conditioned, and processed to determine values for the physiological parameters being measured.
In general, it is desirable to provide non-invasive monitoring of vital signs. While invasive systems are sometimes used, surgery is required to introduce sensors of invasive types into the subject""s body. The sensors typically have leads which emerge from the subject""s body through a fistula. The fistula can provide a pathway for infection.
As an example of non-invasive monitoring of a vital sign, blood oxygen saturation may be measured by providing a small clip-on sensor which includes one or more light sources and one or more light detectors. Variations in the oxygen saturation of the subject""s blood cause resulting variations in the intensity of light reaching the detector. These variations are superimposed upon a variation in the intensity of light reaching the detector which results from the subject""s heartbeat pulses. A device equipped with this type of sensor can also be used to measure pulse rate. Various such devices are known.
One type of system for measuring a subject""s blood pressure relies upon the fact that the speed at which pulse waves propagate through a blood vessel is dependent upon blood pressure. Consequently, if one detects the arrival of a pulse at two different points of a subject""s circulatory system there will be, in general, a difference in the time at which the pulse wave arrives at the two points. This time difference varies according to the blood pressure. One system for measuring blood pressure as a function of such a time difference is described in PCT application No. PCT/CA00/010552, and in the commonly owned and co-pending application entitled CONTINUOUS NON-INVASIVE BLOOD PRESSURE MONITORING METHOD AND APPARATUS which is being filed simultaneously herewith, both of which are fully incorporated herein by reference. Such systems require at least two sensors, one for detecting the arrival of the pulse wave at each of the two points. This type of device may use sensors of the same type as are used to detect oxygen saturation although other types of sensor could also be used.
One problem with such vital signs monitoring equipment is that the accuracy of measurements obtained can depend upon the stability of the signals received from the sensors. Artifacts may be caused by movement of the subject. In the worst case, a sensor may become disconnected from the subject and monitoring may be interrupted until the sensor is replaced.
There is a need for cost effective methods and apparatus for monitoring one or more vital signs of a subject which provide improved accuracy and are affected less by artifacts than current vital signs monitoring equipment.
This invention provides an apparatus for monitoring one or more vital signs of a subject by using a number of sensors. Each sensor originates a signal, typically a pulse signal. A selection system determines performance criteria for a number of groups of signals. Each group includes one or more signals. The apparatus computes a estimated value for a vital sign by either computing a value from each group of signals and taking a weighted average with weights based upon the performance criterion or by selecting one of the groups of signals for which the performance criterion is best and computing the output value from that group of signals.
The output values are relatively insensitive to artifacts and to errors caused by the disconnection or malfunctioning of one sensor. Further advantages and features of the invention are described below.